Energy conservation efforts have focused attention on the reduction of ventilation (exhaust) air where possible. The typical chemistry fume hood used in research laboratories of petro-chemical and pharmaceutical corporations as well as university and high school classrooms is an obvious target. Large volumes of air are heated and cooled only to be discharged through the fume hood. Reduction or elimination of exhaust air during unoccupied hours is a key energy conservation objective.
Fume hood exhaust systems are provided to facilitate researchers and technicians conducting work on noxious or toxic substances without degradation of the room environment. Any attempts to conserve energy must not compromise the integrity of the fume hood device or any surrounding associated devices.
In conventional systems, room air is exhausted outside the building through fume hoods by an exhaust fan or fans. These fans typically run continuously resulting in the wasteful discharge of heated or cooled room air during unoccupied periods.
Some building operators have simply turned off exhaust fans when hoods are not in use, resulting in some energy savings, but usually not without some undesirable side effects. First, most fume hoods have significant quantities of chemicals stored within, even when the hood is not in use. The amount, type and manner in which they are kept can present a hazard to the room environment if flow through the fume hood is totally eliminated. In addition, unless airflow status information is provided at the hood, the operator may not be aware of the lack of exhaust flow through the hood.
Stopping only the exhaust fan without making corresponding adjustments to the supply system results in air imbalance within the building. When the room side supply remains constant, pressurization causes crossflow to other room areas and corridors. This is undesirable not only from a draft standpoint, but also in that it fosters contamination and odor problems. Some imbalance conditions can become so extreme that doors are difficult to open or close.
Shutdown of the exhaust fan without coincident reset of the amount of outside air may result in achieving partial savings of the full potential available. The exhaust fan horsepower would be saved in this case, but the larger savings resulting from reduction of temperature and humidity treatment of the make-up air may be lost.
The installation of a damper control in the exhaust duct which could throttle or reduce exhaust flow through the fume hood in response to a manual or automatic signal has been proposed. This approach has the advantage of reducing exhaust flow from each individual fume hood. Weaknesses of this approach include (1) instability of performance of the exhaust fan due to imbalance, (2) chemical degradation of the damper and operator parts exposed to the exhaust air stream, (3) potential exhaust fan horsepower savings are lost, and (4) the pressure balance within the room is upset since exhaust air flow is altered without corresponding supply air adjustments.
It has also been proposed to use means for stabilizing the performance of the exhaust fan while varying the amount of room air being exhausted. It still requires, however, the use of a balance damper in the exhaust air stream subject to corrosive destruction. Further, potential fan horsepower savings continue to be lost and the pressure balance problems within the room have not been overcome.